1. Field of the Invention
The invention relates to a kitchen appliance for automated preparation of food products, especially, but not limited to, chilled desserts. This appliance can cycle through heating, refrigeration, freezing and mixing functions without having to remove a detachable container from the machine. All appliance functions are controlled by a powerful programmable computer circuit located internally in the device. Food products that can be prepared and stored in the automatic food preparation device include: ice creams, sherbets, soft yogurts, jellos, puddings, custards, milk shakes, sorbets, frappes, frozen juices, mousses, souffles, jams and jellies, soft baby foods, poached fruits, chilled soups, cranberry and blueberry salads, gelatin salads, pasta sauces, seafood sauces, poultry sauces, hollandaise sauce, bernaise sauce, gravy, chili, stews, scrambled eggs, wild rice salads, waldorf salads, potato salad, crab and shrimp salad, hot cereals, deep fried foods, popcorn, pie crust, bread dough, doughnut dough, pizza dough, chocolate candy, fudge, cake frostings, pie fillings, and food products requiring pasteurization.
2. Background of the Related Art
In the past, kitchen appliances have been designed to perform one or more of the functions of heating, cooling, and mixing. For example, several companies produce automatic breadmakers which both knead and bake bread. The American Gas Association is developing a gas tabletop oven that is capable of both heating and cooling. However, a single appliance has not been developed which permits automated preparation of food products involving heating, cooling, and mixing according to a programmed recipe wherein human intervention during food preparation is unnecessary or significantly minimized.
Since the early 1980's, some Italian companies such as Simac have designed ice cream makers with built-in chilling systems. U.S. Pat. Nos. 4,535,604, 4,538,427, 4,573,329, and 4,681,458 all to Cavalli, disclose ice cream making machines that perform chilling and mixing. However, none of these ice cream makers include means for heating. Appliances are generally not constructed which perform heating and cooling functions because refrigeration systems cannot withstand high temperatures. For example, low-level chemical decomposition of R-12 refrigerant begins to take place at approximately 250.degree. F. Raising the temperature of R-12 refrigerant from 70.degree. F. to 200.degree. F. will result in a 500% increase in the pressure of the refrigerant. These high pressure levels in the refrigeration system can result in the failure of solder joints. High heat also increases the chance of compressor failure. Furthermore, R-12 refrigerant reaches its "critical point" at a temperature between 220.degree. F. and 233.degree. F. Above the "critical point", the refrigeration system ceases to function. Additionally, the efficiency of the refrigerant decreases as the "critical point" temperature is neared. Non-CFC refrigerant replacements for R-12 such as R-134A have even less chemical stability than R-12, thus producing even more of a problem with loss of refrigerant at high temperatures.
For several years, combined heating and chilling baths have been available as laboratory apparatus. The major application for these laboratory refrigerated baths is to provide direct temperature measurement and control in an external closed-loop system. Pressure and/or suction pumps located in the refrigerated bath housing circulate liquid around external laboratory apparatus, such as electrophoresis setups. One such apparatus is the Cole-Parmer.TM. miniature refrigerated bath available from Cole-Parmer.TM. Instrument Company of Chicago, Ill. The bath has a built in refrigeration compressor and further includes an immersion heating and cooling coil that extends into a stainless steel bath tank designed to hold water or a silicone bath oil. The Cole-Parmer.TM. miniature refrigerated bath has an operating temperature range of -20.degree. to 100.degree. C. Cole-Parmer.TM. Instrument Company also makes the Cole-Parmer.TM. Polystat.TM. refrigerated bath, which has an operating temperature range of -20.degree. to 200.degree. C.
Another type of laboratory temperature control bath is the circulator bath. This type of bath has an automatic heater and an immersion coolant circulating coil protruding into the cavity that forms the bath tank for holding water, silicone oil or another heat transfer fluid. An external chiller system can be connected to the coolant coil allowing the liquid in the bath to be chilled. A pump circulates the liquid within the bath tank and externally, if desired. The Cole-Parmer.TM. Polystat.TM. Circulator Baths are examples of this type of laboratory apparatus. These baths refrigerate indirectly through the use of an intermediate single phase fluid such as methanol or glycol. The single-phase fluid flows by pump from a refrigerated holding tank located in the external chiller through cooling coils in contact with the bath tank. This indirect chilling method is thermally inefficient since the refrigerant does not directly cool the bath. Furthermore, single-phase intermediate fluids have heat transfer coefficients that are low in comparison to two phase refrigerants such as R-12 or R-134A. The methanol or other intermediate heat transfer fluid must be chilled separately in a heat exchanger before it can cool the bath. Several thermal resistance points are created by this process. Another disadvantage of the Cole-Parmer.TM. circulating baths is that an external intermediate fluid holding tank, refrigeration system, heat exchanger, and pump are required for system operation. The size, weight, mechanical complexity, and manufacturing cost of the refrigeration system are increased by the added refrigeration system components. Furthermore, the Cole-Parmer.TM. refrigerated or circulating baths disclose no means by which recipe ingredients could be mixed within the refrigerating or heating bath. The different types of laboratory liquid temperature control baths are neither designed nor used for food processing applications. Also, the baths are not constructed so they can easily be removed from the refrigeration unit. A detachable foodstuff container is necessary for convenience and sanitation reasons. Furthermore, immersion type coils that cannot be easily removed are not suitable for use with foodstuffs. Immersion coils protruding into a container would make effective mixing of recipe ingredients difficult. Cleaning the container and coil assembly after food preparation would also be a difficult task.
Several detachable container designs for use in ice cream machines have been advanced by the Italians and Japanese. Patents 4,775,233 (Kawasumi et al) and 4,827,732 (Suyama et al) each freeze the detachable container in place, creating an ice gap between the wall of the cylindrical shaped evaporator and the container. Ice is a poor thermal conductor, thus resulting in inefficient cooling of the detachable container. To remove the detachable container from the evaporator assembly, hot gas must be channeled from the compressor back through the evaporator to melt the ice holding the container in place. This design is poor in thermal performance, expensive to manufacture, and inconvenient to the consumer. Furthermore, as in the case of all prior art ice cream machines, any heating of recipe ingredients must be done on a stove before the ingredients can be transferred to the ice cream making machine for freezing.
Evaporator designs used in prior art refrigeration devices also leave room for substantial improvements. Existing evaporator designs use thin metal construction wherein an evaporator coil is fastened to the outer wall of the evaporator assembly. The evaporator assembly is a cylindrical shaped well that accepts a detachable container. Such construction provides inferior thermal performance in comparison to the present design because only about 25% of the round coil surface actually contacts the thin metal. Additionally, prior designs could not be manufactured with precise uniformity from piece to piece because metal distortion occurred when the evaporator coil was soldered or brazed to the outer metal skin. Even a minute amount of metal distortion in these prior art devices increased thermal resistance by creating air or ice gaps. Additionally, the distortion prevented the detachable container from coupling properly with the evaporator assembly, making insertion and removal difficult.
A few prior art designs, of which Cavalli U.S. Pat. No. 4,573,329 is a typical example, made use of elastically formable evaporator assemblies to improve contact with the detachable container. With this design, a mechanical system allows the evaporator assembly to be loosened or tightened around a detachable container. Cavalli U.S. Pat. No. 4,573,329 used a series type manifold refrigerant flow tube pattern that required several connection joints using low-temperature sealant. The attachment of the complex manifold related tube network onto the outer metal wall surface of a cylindrical shaped metal surface to create an evaporator is a difficult and time consuming assembly procedure. The only practical method of attaching the tube network to the metal skin is by using thermal conductive epoxy. This creates a very large thermal resistance point, lowering potential performance. To achieve the elastically formable design, a complex mechanical system consisting of flanges, seats, pistons, pins and cams is used to loosen and tighten the evaporator around the detachable container. Springs are employed to maintain tension. Because of this design, over time and heavy usage, the contact junction with the removable container can be expected to loosen as the mechanical system weakens. Second, in conjunction with the movement related stress placed on the refrigerant flow coil connection joints as a result of the mechanical system' s operation, the chance of refrigerant leaking at the connections from stress cracks is increased.
Prior art evaporator designs were also incapable of maintaining uniform temperatures throughout their evaporator coils. These designs, of which Cavalli U.S. Pat. No. 4,681,458 is a typical example, include a single coil into which cold refrigerant enters at the top. The refrigerant flowing through the coil takes on heat and leaves the base of the evaporator at a higher temperature. Since the refrigerant at the base has less capacity to carry away heat, the area being cooled will be kept colder at the top than at the bottom in the prior art devices.
As mentioned earlier, Cavalli U.S. Pat. No. 4,573,329 suggested the use of a manifold refrigerant flow network for application with elastically formable designs. This design consists of a plurality of lengths of tubing connected together in series by means of two manifold bodies spaced apart in a circumferential direction. The lengths of tubing connected to the manifold are arranged above one another in contact. This design creates an uneven flow of refrigerant through the tube network as the refrigerant pressure drops along the length of the manifold network.